This invention relates to liquid crystal and other electronic displays.
Commercially, it is highly desirable for an electronic display to be as thin and light as possible while still maintaining a high degree of ruggedness and imperviousness to forces that are a consequence of shock and drop. In the area of mobile electronics, such as cell phones and personal digital assistants (PDAs), size and weight are critical factors to the commercial success of a product, but currently breakage of the displays within these devices remains the primary cause of repairs and product returns. In addition, the need for electronic displays that can actually be bent has been acknowledged in several areas: so-called xe2x80x98electronic paperxe2x80x99 in which fiber paper is replaced with a display would be much more compelling as a product if the electronic display could be rolled up or folded like traditional paper; wearable electronics such as computers or multifunction watches would be much more comfortable to the wearer if the display were to conform to the user""s body; chip cards which have strict flexure life-test performance standards would be able to incorporate flexible displays and still conform to those standards. Replacement of the glass substrates within displays with plastic film has been an area of active research within the display community for a number of years.
Electrophoretic displays achieve images via electrophoreticsxe2x80x94the rapid migration of microparticles in colloidal suspensions. Light scattering particles are moved within a dyed colloidal suspension by electrostatic forces. The particles will either move toward the viewer, in which case, the typically white particles are seen by the viewer, or to the surface away from the viewer, in which case, the white particles will be hidden by the dark dye.
Cholesteric displays are another display technology being attempted on plastic substrates. When sandwiched between conducting electrodes, cholesteric liquid-crystal material can be switched between two stable statesxe2x80x94the so-called focal conic and planar statesxe2x80x94in which the liquid crystal""s helical structures have different orientations. In the focal conic state, the helical structures are unaligned and the liquid crystal is transparent. In the planar state, the helical structures"" axes are all perpendicular to the display""s surface resulting in essentially monochromatic transmission by the display.
The Gyricon display being developed by Xerox, is made of microscopic beads, randomly dispersed and held in place between two plastic sheets by a flexible elastomeric matrix of oil-filled cavities. The balls have strongly contrasting hemispheres, black on one side and white on the other. The white side is highly reflective, while the black side absorbs light. Each hemisphere has a unique intrinsic charge, resulting in a force on the ball when an electric field is applied and the axis of the ball isn""t aligned with the field. The side of the ball presented for display depends on the polarity of the voltage applied to the electrode. In all three of these cases, while they have some positive features such as high contrast and compatibility with plastic substrates, they all currently high drive voltages, have slow response times, and are not compatible with commercially available drive electronics.
Liquid crystal displays (LCDs) are attractive because of the low drive voltages required to switch them, their relatively fast response times, the wide availability of drive electronics, and the significant intellectual and manufacturing investment in the technology. Attempts have been made to develop LCDs that intermixed the liquid crystal within a polymer matrix in order to make them compatible with plastic substrates, one example being polymer dispersed displays (PDLCDs). PDLCDs are fabricated by intermixing the liquid crystal and a prepolymer into a solution prior to assembling the display. After assembling the display, the polymer is cured, typically by ultraviolet light. During the polymerization the LC separates out from the polymer into microscopic droplets. Since the droplets of LC are not in contact with any alignment layer, the orientation of the molecules is random and light is scattered by the droplets. Applying a voltage to the electrodes of the PDLCD causes the LC molecules to become aligned, resulting in the display becoming transparent. Like the other flexible displays, PDLCDs required high drive voltages not generally compatible with existing drive electronics. Prior art such as U.S. Pat. Nos. 4,688,900, 5,321,533, 5,327,271, 5,434,685, 5,504,600, 5,530,566, 5,583,672, 5,949,508, 5,333,074, and 5,473,450 all make use of phase separation of an LC/polymer mixture during polymerization of the polymer using light as the curing mechanism (photopolymerization).
Methods have been developed to achieve anisotropically dispersed LC/polymer structures which might have drive voltages lower then those achieved in PDLCDs. U.S. Pat. No. 5,949,508 describes a method in which a lamellar structure is achieved whereby the LC and polymer are disposed on opposite substrates; this reduces the drive voltages necessary to switch the device, but results in a structure where it is only practical to have the rubbed alignment surface on one of the substrates. While this structure is effective with nematic or electrically controlled birefringence (ECB) displays, it becomes more difficult to construct displays such as twisted nematic (TN) and super twisted nematic (STN) which typically require alignment surfaces on both substrates. U.S. Pat. Nos. 5,473,450 and 5,333,074 describe methods of localizing the polymer during photopolymerization by exposing only portions of the device to the light source using masks. Polymer structures of a size on the order of a pixel (xcx9c0.3 mm) are achievable, but manufacturing may be more difficult since the photomask must generally be aligned to the electrode structure within the device and expensive collimated UV light sources must generally be employed. Structures much smaller than 0.3 mm may be difficult to achieve due to the inherent scattering of the LC/polymer mixture. U.S. Pat. No. 5,473,450 teaches the patterning of photoinitiator onto the alignment layer, but this method generally requires a highly accurate, screened deposition of the chemical photoinitiator onto the substrates. Proper alignment of the silk-screening mask to the clear ITO electrodes may be difficult to achieve, and the introduction of chemicals directly onto the polyimide alignment surface may result in poor alignment of the LC to the alignment surface, poor appearance of the display and lower manufacturing yields.
Other methods have been developed for providing adhesion between plastic substrates involving adhesive elements. In one method, a thermoset (e.g., epoxy) or thermoplastic (e.g., hot melt) adhesive as a solid coating around the spacer elements (U.S. Pat. Nos. 4,678,284, 5,130,831). Heat and pressure are applied after the coated spacers are in place between the substrates. Other methods use solid adhesive elements separate from the spacer elements (U.S. Pat. Nos. 5,812,232, 5,581,384, 6,004,423).
In addition to the breakage problems due to shock and drop, glass substrate displays also have difficulty surviving extremes of temperature. When the temperature of a display is cycled between cold and hot it will sometimes develop small voids between the spacers and the liquid crystal fluid. While the voids are small in size, they typically are noticeable enough that the display will be returned for repair. The voids are due to the mismatch in the thermal coefficients of expansion between the LC and the typically glass or plastic spacers. When a glass substrate display is assembled at room temperature and then sealed, its volume is essentially fixed at that point. As the display is cooled down, both the LC and spacer material will contract but due to the mismatch in the thermal coefficients of expansion and the mechanical discontinuity at the spacers, stress is localized around the spacers and voids develop. Initially, the voids are small areas of vacuum or very low pressure, but the more volatile components of the LC quickly move to a gaseous phase to fill the void to achieve a lower energy equilibrium state. When the display is returned to room temperature, the vapor filling the voids prevents the voids from being absorbed back into the LC, and the damage is typically permanent. Display manufactures have solved this problem by, amongst other methods, utilizing specially fabricated spacers that have a softer, more compliant exterior coating surrounding a core of either glass or plastic. The outer compliant layer acts to relieve the stresses encountered during thermal cycling of the display, thus preventing the voids. Because of the difficulty of manufacturing these spacers, they are often 10-20 times more expensive than regular spacers and so are often used only when absolutely necessary.
In general, in a first aspect, the invention features a liquid crystal display device comprising two substrates facing and spaced from each other, at least one of the substrates being transparent, electrodes positioned to establish an electric field in the space between the two substrates, one or more spacer elements located between the substrates, the spacer elements having been introduced between the substrates during assembly of the device, an electrooptic material filling at least a portion of the space between the two substrates, and a polymeric material filling at least a portion of the space between the substrates, the polymeric material comprising a liquid prepolymeric material that was applied to the spacer elements in liquid form and having been polymerized in situ after the spacer elements were in place between the substrate.
In general, in a second aspect, the invention features manufacturing a liquid crystal display device by introducing spacer elements between two substrates that face each other, at least one of the substrates being transparent, applying a liquid prepolymeric material to the exterior surfaces of one or more spacer elements before or after introduction of the spacer elements between the substrates, positioning electrodes to establish an electric field in the space between the two substrates, filling at least a portion of the space between the two substrates with an electro-optic material, and polymerizing the liquid prepolymeric material in situ to form solid polymeric material filling at least a portion of the space between the substrates.
In preferred implementations, one or more of the following features may be incorporated. The polymeric material may be in the vicinity of the spacer elements. The liquid prepolymeric material may be applied to the spacer elements prior to their introduction between the substrates. The liquid prepolymeric material may be applied to the spacer elements after their introduction between the substrates. The liquid prepolymeric material may be encased in a collapsible shell surrounding at least some of the spacer elements. The prepolymeric material can constitute one or more of the following materials: monomers (e.g., basic polymer chain material), oligomers (e.g., for cross linking), inhibitors (e.g., to prevent polymerization during storage), adhesion promoters, and polymerization initiating or enhancing (PIE) materials. The liquid prepolymeric material may have a viscosity equal to or less than 2,000,000 centipoise. A polymerization initiating or enhancing (PIE) material may be brought into contact with the liquid prepolymeric material. The PIE material may be brought into contact with the liquid prepolymeric material in one of the following ways: it is mixed with the liquid prepolymeric material applied to the spacer elements; it is carried on or within the spacer elements; it is dissolved or suspended in the electrooptic material. The liquid prepolymer material and the PIE material may both be encased in a collapsible shell surrounding at least some of the spacer elements. The polymerization in-situ may comprise initiating polymerization by application of light. The liquid prepolymeric material may be a thermoset material, and the polymerizing in situ may comprise the application of heat. The polymeric material may comprise polymer supports that extend between the two substrates. The polymeric material may comprise polymer members that do not extend between the two substrates. Additional spacer elements without prepolymeric material may be introduced between the substrates. The spacer elements may comprise a large number of generally spherical or cylindrical elements. The spacer elements may comprise glass. The glass may be etched. The spacer elements may comprise plastic. The plastic may be porous. The spacer elements may comprise high-surface area particles that are nanoporous, mesoporous, or microporous. The spacer elements may be randomly located in the space between the substrates. The spacer elements may comprise a large number of elements generally of smaller diameter than the space between the substrates. The spacer elements may comprise a large number of elements randomly positioned across the space between the substrates. The spacer elements may generally not be in contact with the substrates. The spacer elements may be in contact with only one substrate. The spacer elements may comprise a lattice network structure. The lattice network structure may be two-dimensional. The lattice network structure may be three-dimensional. The spacer elements may be non-uniform in size and shape. The spacer elements may have a rough surface. Most of the spacer elements may be free to move around in the space between the substrates prior to polymerization. A porous membrane may serve as a spacer element. The porous membrane may be an extensible porous membrane. The spacer elements may be located in non-image areas of the substrate. The spacer elements may be located along the peripheries of the substrates and serve as one or more sealing members sealing the space between the substrates. The spacer elements may be located at interpixel regions. The prepolymer may contract upon in situ polymerization. The majority of the polymer supports may be bonded to each of the two substrates. The polymer supports may be primarily separate members not interconnected with one another. One or more interconnecting regions of polymer may interconnect a majority of the polymer supports. One of the interconnecting regions may comprise a layer of polymer adjacent one of the substrates. The spacer elements may be dry sprayed on to the substrate before application of the electrooptic material. The spacer elements may be wet sprayed on to the substrate. A solvent may be used for wet spraying comprises the prepolymeric material in solution or suspension. The PIE material may comprise one or both of the following: an initiator and an accelerant of the in situ polymerization process. The PIE material may be light activated. The PIE material may comprises a photoinitiator. The photoinitiator may comprise a plurality of photoinitiators of different spectral sensitivities, so that polymerization may be initiated at different times in different locations. The light may be ultraviolet light. The PIE material may be heat activated. The PIE material may be self-activated after a period of time following assembly of the display. The PIE material may comprise both a photoinitiator and an accelerant. The prepolymeric material may applied to the substrates by at least one of the following processes: pipette, syringe, or printing. The printing may comprise a silk screen, gravure, flexographic, or lithographic process. The spacer elements may be porous structures with a porous matrix, and the prepolymeric material may absorbed into the porous matrix of the porous structures. The porous structures may be nanoporous ceramic or silica based materials. The spacer elements may comprise an open network of polymer spheroids formed so that the electrooptic material fills inter-polymer regions. The porosity of the porous structure may be selected to yield a desired adhesion of the spacer element to a polymer matrix comprising the in situ polymerized material. The electrooptic material may be a liquid crystal material. The electrooptic material may be a mesomorphic material. The invention may further comprise at least one electrode on at least one substrate to generate the electric field and at least one electrode on the second substrate. The prepolymeric material may comprises at least one of the following: acrylic-based adhesive, epoxy-based adhesive, urethane-based adhesive. The prepolymeric material may be primarily cured by application by one of the following: light, heat, intermixing of a chemical additive. The substrates may comprise a flexible polymer material. The display may be capable of withstanding the flexing text referenced in the detailed description.
Two important specifications that impact a plastic display""s durability are its compressive and peel strength. In addition, when spacer elements are a single size and are in contact with both substrates in the assembled cell, it is often the case that the compressive strength is achieved with a lower spacer density than peel strength. It is therefore desirable to be able to independently improve a display""s compression and peel strength while reducing contrast degradation. Spacer elements composed of the same material as the surrounding polymer provide the significant benefit of additional polymer interconnections between the substrates resulting in added peel strength but with relatively little impact on compressive strength. By adjusting the relative densities of the spacers and the polymer spacers and the durometer of the polymer spacers, improvements in compressive and peel strengths of the display device""s laminate structure can be achieved.
Use of liquid adhesives provides a number of benefits over solid adhesives such as hot melts or thermoplastic adhesives. Liquid adhesives can achieve significantly better surface adhesion, elongation before break, and tensile strength, for instance, than solid adhesives; these performance parameters are all important factors determining how durable the adhesive bond will be.
Other features and advantages of the invention will be apparent from the following detailed description and from the claims, and from the disclosure and claims of my applications entitled xe2x80x9cElectrooptical Displays with Polymer Localized in Vicinities of Substrate Spacers,xe2x80x9d Electrooptical Displays Constructed with Polymerization Initiating and Enhancing Elements Positioned Between Substrates,xe2x80x9d and xe2x80x9cElectrooptical Displays with Multilayer Structure Achieved by Varying Rates of Polymerization and/or Phase Separation,xe2x80x9d filed on even date herewith (and incorporated by reference).